Crankcase bolt

Ive had an oil leak for some months now on my FF 1000 which has got bad enough to now need attention. I’ve identified it as coming from one of the crankcase bolts going up above the exhaust balance box. Unfortunately the thread has been stripped in the crankcase so my question is - what are those spiral tapers on the shank of the bolt ? They look like they are for oil to flow round. My reason for asking is I’m hoping I might be able to get a longer standard bolt so I can get to the good thread further up the tunnel and get a fix.

The bolt pictured has no oil flowing around it,the flutes allow it to be a very close fit in the crankcase and helps to keep the two halves aligned.
It is questionable if there is any spare thread above the length of the standard bolt.
I had a similar problem with the 500 cases.
I had the advantage of 1 the cases being split and 2 the crankcase bolts being M8 rather than the M9 used on the 1000.
If you can find an M9 helicoil kit there will be a way to sort this in situ.
Regards Bif

The spirals could be for controlled stretch like the CB900F bikes.
A timesert is perhaps a better way of fixing a critical thread in an engine than the helicoil repair.

hi martin,

ive just been having a look at a couple of knackered crank cases i have in the garage, i would say that you probably have a about 7mm of threaded hole left on the 2 outer bolts and about 12mm on the inner bolts,
I dont know how easy it will be to get hold of a longer M9 bolt or a helicoil

If it is one of the outer bolts i would consider drilling the lower half of the crank case out to m10, then drilling through into the upper casing with a 9mm drill, then carefully working a m10 tap into it… and using a M10 bolt with lots of gasket seal on the shaft :-[ i wouldent try this on the inner bolts as they run through pretty close to the main bearings.

Thanks people and especially Pete for a comprehensive reply, very helpful. Knowing that that spiral has probably a “stretch” or “filling” purpose I will use a thin piece of wire with 2mm of its tip turned at 90’ I should be able to find out exactly / roughly if I have any unused thread beyond the length of the bolt itself. I can see exactly what part of the bolt is in contact with the crankcase.
If I am able to sort this in this relatively simple way I hope to see you at Hulme End on Saturday evening on it. If not then I’ll be on my Benelli.
Many thanks